Granular urea is commonly used as a source of nitrogen in chemical fertilizer. Methods of manufacturing urea fertilizer as well as methods of processing fertilizer into particles via prill and/or granulation techniques are well known. In general, urea is produced commercially by the basic reaction of ammonia with carbon dioxide at elevated temperatures and pressures. Exemplary commercial applications are described in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 23 (1983) pgs. 551-562. As described, the reaction products from such commercial processes are normally 70 to 80% aqueous urea solutions which can subsequently be processed to form solid urea particulates, such as granules. See Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 23 (1983) pgs. 562-572. The resulting urea particles/granules produced therefrom exhibit properties of caking and friability. The caking tendency causes the initially prepared, free-flowing product to change into a dense, solid, essentially singular mass in the storage pile. The friability property refers to the tendency of the urea granules to break easily into smaller particles. As a result, substantial amounts of dust can be generated as a result of handling, transporting, and applying the urea granules to the soil environment. Modern chemical fertilizer application equipment often includes rotating turbine fan type field distribution equipment which can result in breakage of the urea granules. As a result, the smaller particles fall in a shorter trajectory from the fertilizer distributor and form a more narrow and more concentrated swath on the soil surface than originally intended. In addition, the generated dust may cause health problems.
Numerous techniques have been developed for addressing the caking and friability properties of urea. One technique involves the addition of a small amount of formaldehyde as a conditioning agent. In general, formaldehyde is added in a concentration of between about 0.1% to 0.5% by weight and reacts with the urea to form urea-formaldehyde reaction products. While the addition of formaldehyde to urea modifies the caking and friability properties to desirable levels, formaldehyde has been designated as a toxic and/or carcinogenic material by the U.S. Environmental Protection Agency (EPA). This designation may lead to the restriction or total prohibition of the use of formaldehyde in agricultural products, including its use as a conditioning agent in the preparation of urea for fertilizer.
Urea-formaldehyde was developed as an alternative conditioning agent. The following patents discuss the reaction of formaldehyde with urea to form urea-formaldehyde products: U.S. Pat. No. 3,112,343 to Elstrom, et al.; U.S. Pat. No. 4,160,782 to Van Hiatte, et al.; and U.S. Pat. No. 4,204,053 to Allgeuer, et al. Health and safety considerations, however, have limited the use of formaldehyde in fertilizer products. A risk associated with the use urea-formaldehyde conditioning agent is related to storage and handling prior to blending with molten urea due to free formaldehyde in the conditioning agent.
Anti-caking properties have been imparted to fertilizer particles by utilizing clay, talc, and/or surfactants to coat the particles. Additionally, various oils are sprayed on the particles for controlling dust.
Another technique for modifying the caking and friability properties of urea is to treat the urea with lignosulfonates as described in U.S. Pat. No. 4,587,358 to Blouin, and by treating urea with modified lignosulfonate as described by U.S. Pat. No. 5,032,164 to Sanford et al.
Techniques have been developed for using polylactic acid polymers in controlled release applications including agricultural and waste management applications. See U.S. Pat. No. 5,563,238 to Bonsignore et al. and U.S. Pat. No. 5,360,892 to Bonsignore et al. Techniques have been developed for providing granular fertilizer with a decomposable coating. See U.S. Pat. No. 5,176,734 to Fujita et al. which describes using poly-3-hydroxy-3-alkylpropionic acid as a component of a decomposable coating for granular fertilizer.